Dynamics of Coupled Quantum Spin Chains

Static and dynamical properties of weakly coupled antiferromagnetic spin chains are treated using a mean-field approximation for the interchain coupling and exact results for the resulting effective one-dimensional problem. Results for staggered magnetization, Néel temperature, and spin wave excitations are in agreement with experiments on ${\mathrm{KCuF}}_3$. The existence of a narrow longitudinal mode is predicted. The results are in agreement with general scaling arguments, contrary to spin wave theory.